A light emitting element formed using a light emitting material has characteristics of being thin, lightweight, high-speed response, direct current low voltage drive, and the like. Therefore, there are high expectations for its application in next generation flat panel displays. Further, since a light emitting device in which light emitting elements are arranged in a matrix form has a wider viewing angle as compared with a conventional liquid crystal display device, the light emitting device has excellent visibility.
A light emitting mechanism of a light emitting element is shown below. Upon applying voltage to a light emitting layer interposed between a pair of electrodes, electrons injected from a cathode and holes injected from an anode are recombined at a center of light emission of the light emitting layer to generate molecular excitons. Then, since the molecular excitons emit light energy upon returning to a ground state, light emission is generated. As excited states, a singlet excited state and a triple excited state are known, and light emission can be generated through either the singlet excited state and the triplet excited state.
A light emission wavelength of a light emitting element is determined by a bandgap of a light emitting molecule contained in the light emitting element. Therefore, by adjusting structures of light emitting molecules, light emitting elements having various types of light emission colors can be obtained. Further, by using light emitting elements capable of emitting light with red, blue, and green, which are three primary colors of light, a full-color light emitting device can be manufactured.
In order to manufacture a full-color light emitting device having excellent color reproducibility, red, green, and blue light emitting elements each having high reliability along with excellent color purity are required. As a result of development of materials in recent years, as for red and green light emitting elements, high reliability and excellent color purity have been already achieved. However, as for a blue light emitting element, sufficient reliability and excellent color purity have not been achieved yet. In order to solve this problem, many researches have been done in a blue light emitting element (e.g., see patent document 1).
[Patent Document 1]: Japanese Patent Application Laid-Open No. 2003-31371